Sorting machines are known that transfer individual letters in succession and that include weighing machines arranged along the path, in such a way as to measure the weight of the moving letters. The weight measured, for example, is used for verifying whether the rates paid are correct with respect to the physical characteristics of the letters.
The detection of the weight imposes waiting for a stabilization of the value measured by the sensors of the weighing machine during the passage of each letter, before reading or acquiring said value. For sorting machines that reach a rate of 50 000 letters per hour, known weighing machines do not manage to measure the weight in a precise way during the passage of the letter on the weighing machine in so far as the time interval of passage available for the measurement is so small (even 35·10−3 s) as not to allow optimal stabilization of the value measured by the sensors of the weighing machine.
In order to manage to carry out weighing with a sufficient precision, known weighing machines have a limit capacity of approximately 18 000 letters per hour. Consequently, it is necessary to provide two weighing machines that operate in parallel to reach at least a rate of 36 000 letters per hour. The fact of using two weighing machines in parallel increases the costs of the equipment and makes it necessary to install a switch section upstream and a confluence section downstream of the weighing machines, with consequent risks of interference or bottlenecks in the flows of letters that reach the confluence section, with consequent increase also in the costs of operation. In addition, this installation occupies a significant space, in particular of approximately 10 m2, due to the length of the weighing machine (2 m) times the width of the path of the letters comprising the operative spaces around the sorting line (5 m).
In addition, in known weighing machines of the type described above the tare is defined by a horizontal plate and by a conveyor that is carried by said plate and conveys the letters. Said tare significantly affects the measurement of the sensors with respect to the weight of the letter so that the sensors must have a full scale range that is very wide as compared to the effective weight of the letter that is to be determined, and, consequently, the measurement has a relatively low precision.